This invention relates to a process for removing arsenic containing compounds from wastewater, groundwater, and the like wherein said compounds are normally present in a soluble or dissolved form. Incidental thereto, the process of this invention also is capable of removing certain heavy metals associated with such wastewater or groundwater streams.
In recent years, emphasis on discharging clean water into rivers, bays, and the like has increased dramatically. Discharge of industrial wastewater containing toxic substances has in some areas polluted groundwater or surface water to an extent forcing the intervention of federal and state instrumentalities. Efforts to clean up the environment relative to such discharges after the fact have met with uncertain success. It is, therefore, desirable to prevent such discharges by treating the industrial wastewater at the source before discharging and also to provide for the treatment of groundwater and the like to remove such contaminants.
It has been known that certain heavy metals, such as chromium, which are normally present in wastewater in soluble form can be readily precipitated and removed from the wastewater flow. In U.S. Pat. Nos. 3,926,754; 4,036,726; and 4,123,339, assigned to the assignee of this invention, heavy metals such as hexavalent chromium ions were effectively removed from cooling tower wastewater electrochemically. The disclosures of these patent are hereby incorporated by reference.
In the above patents, a process and apparatus were described wherein wastewater containing hexavalent chromium ions is caused to flow between a plurality of electrodes. It was discovered that when the anode has a surface or a portion of the surface of iron, an iron alloy or insoluble iron compound, an iron hydroxide derivative will be produced electrochemically. In turn, an insoluble trivalent chromium compound, preferably as the hydroxide will be produced which will complex with or otherwise physically or chemically combine with the insoluble iron derivative to thereby permit removal from solution. Whereas it was previously considered necessary to reduce hexavalent chromium to trivalent chromium in acidic solution, it was discovered that the iron compound or complex formed will reduce the hexavalent chromium and coprecipitate therewith in a solution having a pH of about 4 to about 11. Accordingly, the invention described and claimed in said patents produces an insoluble iron-chromium precipitate without pH adjustment to thereby rapidly and efficiently remove hexavalent chromium from solution. The precipitate is then removed from aqueous media utilizing conventional techniques such as a clarifier, settling pond or the like and the aqueous media thereby clarified is suitable for disposal.
In this process, hexavalent chromium undergoes cathodic reduction to form trivalent chromium as insoluble chromic hydroxide which complexes with iron entering solution at the anode. The products are not susceptible to further electrolytic oxidation at the anode, back to hexavalent chromium, apparently due to the difference in ionization potential at least in part because the production of the hydroxide ion at the cathode occurs at a much lower potential than other electrode reactions. Thus, because of the nonamphoteric state of the iron complex, the reaction continues until the undesirable contaminating ions are completely or substantially removed from solution in the aqueous media.
It was also discovered in U.S. Pat. No. 4,880,510, also assigned to the assignee of this invention, that the electrolytic cell and process could be used to remove color impurities such as dyes from wastewater solution. The ferrous iron generated at the anode reacts with hydroxide ion to form an iron complex or compound which further was found to react with or otherwise remove the color bodies from aqueous media as an insoluble precipitate therewith. Accordingly, the disclosure of U.S. Pat. No. 4,880,510 is also incorporated by reference herein.
Therefore, broadly speaking, soluble metals which require chemical reduction such as from hexavalent to trivalent chromium in order to form insoluble species could be removed by the iron specie generated electrochemically as a coprecipitate or complex therewith. Color bodies similarly reacted to form coprecipitates or at least complexes with the insoluble iron specie generated in the electrochemical cell so that the insoluble complex or combination could be removed from solution using conventional techniques such as flocculation, filtering, a settling pond, or the like. Contaminants in wastewater or groundwater, however, may include a number of heavy metals in addition to chromium which require oxidation in order to form insoluble species or which for a variety of reasons are not readily removable from solution by the electrochemical generation of an iron compound or specie under neutral or slightly basic conditions.
In U.S. Pat. No. 5,013,453, heavy metals are removed from solution using an alkali precipitation technique carried out at a pH of between 9 and 12. The invention uses coprecipitation with ferrous ions added to solution to reduce the heavy metal content below equilibrium concentrations. The ferrous ions are added and then oxidized to ferric at a pH maintained by the addition of a base such as ammonia. Ferrous salts are added to solution to supply the ferrous ion and aqueous ammonia is preferred to be used as an additive to supply hydroxyl ions.
In U.S. Pat. No. 4,566,975, heavy metals such as arsenic are removed in a two step process which involves an alkaline precipitation carried out at a pH of at least about 8 and using ferrous sulfate as an additive.
In U.S. Pat. No. 4,490,257, contaminants are removed from a paper pulping operation using electrolysis with plate electrodes. The electrodes, however, are extremely resistant to corrosion in that chlorine and chlorides are released at the cathode. In U.S. Pat. No. 5,043,080, contaminated groundwaters are treated with hydrogen peroxide and transition metal ions at an acid pH in the presence of ultraviolet light. The main object, however, is the removal of organic contaminants rather than heavy metals.
Also, in U.S. Pat. No. 4,163,716, it was recognized that heavy metals and color bodies from dye house affluent could be removed with ferrous ions supplied by iron electrodes with the ferrous ion oxidizing to the ferric state by use of an oxidizing agent such as hydrogen peroxide. At a pH of between 7 and 9, heavy metals and traces of color adhere to the ferric hydroxide floc which then may be removed. This process also involves a pH adjustment from a reaction pH of below 6.5 to a pH of from 7 to 9 to achieve removal of color particles.
Accordingly, it remains an object to provide a process from the removal of a wide variety of heavy metals and other contaminants from wastewater and groundwater.